This invention relates to archery bows and, in particular, to a limb attachment for a compound bow.
A compound bow differs from a long bow in that a block and tackle mechanism is used to bend the bow: pulleys or wheels are attached at the free ends of the limbs to obtain a mechanical advantage in bending the bow. The limbs can be made approximately three times as stiff as for a longbow of the same draw weight. Eccentrically mounted wheels enable one to use a much higher maximum draw weight because they provide a substantial "let off" or reduction in the holding force of a drawn bow. The combination of stiffer limbs and greater draw weight applies large forces to the components of a compound bow.
A compound bow is typically made in three pieces: an elongated, rigid handle and a pair of limbs. The limbs overlap the ends of the handle a short distance and are cantilever mounted to the ends of the handle. A pocket in each end of the handle receives the end of a limb. Each limb is held in place by a limb bolt passing through a hole or slot at one end of the limb and engaging a threaded bore in the pocket. A semi-circular depression at the outer end of each pocket receives a half-round member having its flat side attached to the underside of the limb. The underside of the limb does not touch the handle but rests on the half-round member, about which the limb can pivot as the limb bolt is tightened or loosened.
As used herein, the "underside" or "inside" of a limb is the generally concave side, i.e. the side facing an archer holding a drawn bow. The "outside" surface of a limb is the generally convex side facing the target.
The distance from the pivot to the free end of the limb is considerably greater than the distance from the pivot to the limb bolt. The large forces in the bow are therefore concentrated at the connection of the limb to the handle, particularly at the pivot. The limb bolt, which is used for adjusting the draw weight of the bow, is not located in a closely fitting hole or slot. Thus, the limb is located by the pivot, where the forces are concentrated.
There are problems with this construction of a compound bow. A first problem is the large shear force on the pivot as the bow is drawn and fired. The adhesive between the pivot and the limb must be able to withstand the longitudinal forces on the limb. A second problem is that the placement of the pivot is critical. Any slight misalignment of the pivot causes the limbs to be out of alignment with the handle, increasing the shear forces on the pivot and possibly causing the limb to rub and wear in the pocket. Thus, the gluing operation is a critical manufacturing step.
Another problem is that the limb bolt is often overtightened by an archer seeking to increase the draw weight of his bow. Overtightening can split the end of the limb or break the pivot by forcing the limb to move longitudinally.
In view of the foregoing, it is therefore an object of the invention to provide an improved limb attachment for a compound archery bow.
Another object of the invention is to simplify the manufacture of compound archery bow.
A further object of the invention is to prevent overtightening of limb bolts in compound bows.
Another object of the invention is to eliminate shear forces on the pivot of a compound bow.
A further object of the invention is to provide a self-centering assembly of limbs in the pockets of the handle of a compound bow.